Which one of the following statements is
incorrect?

Options

A. The residual air in lungs slightly decreases the efficiency of respiration in mammals
B. The presence of non-respiratory air sacs, increases the efficiency of respiration in birds
C. In insects, circulating body fluids serve to distribute oxygen to tissues
D. The principle of countercurrent flow facilitates efficient respiration in gills of fishes

Answer

To determine which statement is incorrect regarding respiration, we will analyze each statement provided in the question.

### Step-by-Step Solution:

1. **Understanding Residual Air**:
- Residual air is the amount of air that remains in the lungs after a forceful expiration. It is important to note that this air cannot be expelled from the lungs.
- **Hint**: Remember that residual air is crucial for maintaining lung structure and preventing lung collapse.

2. **Evaluating Statement 1**:
- The first statement claims that "the residual air in lungs slightly decreases the efficiency of respiration in mammals."
- Since residual air helps maintain a constant exchange of gases and does not negatively impact the efficiency of respiration, this statement is incorrect.
- **Hint**: Think about how residual air contributes to gas exchange rather than hindering it.

3. **Evaluating Statement 2**:
- The second statement states, "the presence of non-respiratory air sacs increases the efficiency of respiration in birds."
- This is a correct statement as air sacs in birds allow for a more efficient respiratory process by providing a continuous flow of air through the lungs.
- **Hint**: Consider how birds have a unique respiratory system compared to mammals.

4. **Evaluating Statement 3**:
- The third statement mentions, "in insects, the circulating body fluid serves to distribute oxygen to tissue."
- This is also correct. Insects have a tracheal system that delivers oxygen directly to tissues, and their body fluid (hemolymph) helps in the distribution.
- **Hint**: Recall that insects do not have a circulatory system like vertebrates; they use a different mechanism for oxygen transport.

5. **Evaluating Statement 4**:
- The fourth statement asserts, "the principle of counter current flow facilitates efficient respiration in gills of fishes."
- This statement is correct as counter current flow maximizes oxygen uptake in fish gills by ensuring that water and blood flow in opposite directions.
- **Hint**: Think about how this principle enhances the diffusion of oxygen.

6. **Conclusion**:
- After evaluating all statements, the incorrect statement is the first one: "the residual air in lungs slightly decreases the efficiency of respiration in mammals."
- Therefore, the answer is **Option A**.

### Summary:
The incorrect statement is: "the residual air in lungs slightly decreases the efficiency of respiration in mammals."